Electrical machines

ABSTRACT

An electrical machine comprises a rotor without windings, a stator having an armature winding ( 24,25 ) and a field winding ( 10 ) for generating a magnetomotive force in a direction extending transversely of the magnetomotive force generated by the armature winding. An electric circuit ( 40 ) is provided for controlling the current in the armature winding ( 24, 25 ) such that periods in which a magnetomotive force in one direction is associated with a first current pulse alternate with periods in which a magnetomotive force in the opposite direction is associated with a second current pulse. A position sensor is provided for monitoring the rotational position of the rotor and for supplying output signals dependent on the speed of rotation of the moor. Furthermore a control system ( 32 ) supplies control signals to the circuit ( 40 ) to control the current in the armature winding ( 24, 25 ), each control signal being produced in response to detection of a respective one of the output signals from the position sensor and being maintained for a length of time determined by the duration of the output signal. Such an arrangement enables control of acceleration, no-load speed and loaded torque-speed characteristics (and braking of the machine) to be achieved with simple on/off control of armature and field switching devices (if present), so that the control circuitry can be produced at relatively low cost. Simplification of the control circuitry is further ensured by the fact that such control can be effected without current sensing.

Reference is also made to the Applicants' co-pending Applications Nos.PCT/GB00/03213, PCT/GB00/03201 and PCT/GB00/03214 the disclosures ofwhich are incorporated herein by reference.

This invention relates to electrical machines, and is concerned moreparticularly, but not exclusively, with electric motors.

FIGS. 1a and 1 b shows a conventional two-phase variable reluctancemotor comprising a stator 2 having two pairs 3, 4 of oppositely disposedinwardly directed salient poles provided with two pairs 5, 6 ofenergising windings corresponding to the two phases, and a rotor 7having a single pair 8 of oppositely disposed outwardly directed salientpoles without windings. Each of the four energising windings is woundabout its corresponding pole, as indicated by the symbols Y—Y denotingtwo diametrically opposite portions of each winding of the winding pair6 and the symbols X—X denoting two diametrically opposite portions ofeach winding of the winding pair 5. An excitation circuit (not shown) isprovided for rotating the rotor 7 within the stator 2 by alternatelyenergising the stator windings in synchronism with rotation of the rotorso that torque is developed by the tendency of the rotor 7 to arrangeitself in a position of minimum reluctance within the magnetic fieldproduced by the windings, as will be described in more detail below.Such a variable reluctance motor offers tho advantage over aconventional wound rotor motor that a commutator and brushes which arewearing parts, are not required for supply of current to the rotor.Furthermore other advantages are provided because there are noconductors on the rotor and high-cost permanent magnets am not required.

The symbols +and − in FIGS. 1a and 1 b show the directions of currentflow in the windings in the two alternate modes of excitation in whichthe rotor 7 is attracted either to the horizontal position or to thevertical position as viewed in the figures. It will be appreciated thatrotation of the rotor 7 requires alternate energisation of the windingpairs 5 and 6, preferably with only one winding pair 5 or 6 beingenergised at a time, and with the current usually being supplied to eachwinding pair 5 or 6 in only one direction during such energisation.However the windings can only be energised for a maximum of half thetime per revolution if useful torque is to be produced, so that highlyefficient utilisation of the electrical circuit is not possible withsuch a motor.

By contrast a fully pitched variable reluctance motor, as described byJ. D. Wale and C. Pollock, “Novel Converter Topologies for a Two-PhaseSwitched Reluctance Motor with Fully Pitched Windings”, IEEE PowerElectronics Specialists Conference, Braveno, June 1996, pp. 1798-1803and as shown in FIGS. 2a and 2 b (in which the same reference numeralsare used to denote like parts as in FIGS. 1a and 1 b) comprises twowindings 10 and 11 having a pitch which is twice the pole pitch of themotor, that is 180° in the example illustrated, and disposed at 90° toone another. The winding 11 may be wound so that one part of the windingon one side of the rotor 7 fills a stator slot 12 defined betweenadjacent poles of the pole pairs 3, 4, and another part of the winding11 on the diametrically opposite side of the rotor 7 fills a stator slot13 defined between two further adjacent poles of the pole pairs 3, 4.The winding 10 has corresponding parts filling diametrically opposedstator slots 14 and 15. Thus the two windings 10 and 11 span the widthof the motor with the axes of the windings 10, 11 being at right anglesto one another.

Furthermore two alternate modes of excitation of such a motorcorresponding to the horizontal and vertical positions of the rotor 7are shown in FIGS. 2a and 2 b from which it will be appreciated thatboth windings 10, 11 are energised in both modes of excitation, butthat, whereas the direction of current flow in the winding 10 is thesame in both modes, the direction of current flow in the winding 11changes between the two modes. Since current is supplied to both phasewindings 10, 11 in both modes and since each winding 10 or 11 occupieshalf the total stator slot area, such a system can achieve 100%utilisation of its slot area. This contrasts with the 50% utilisationachieved with the conventional wound variable reluctance motor describedabove in which only one phase winding is energised at a time.Furthermore, since there is no requirement for the direction of currentin the winding 10 to change, the winding 10, which may be termed thefield winding, can be supplied with direct current without any switchingwhich leads to simplification of the excitation circuit used. Howeverthe winding 11, which may be termed the armature winding, must beenergised with current which alternates in synchronism with the rotorposition so as to determine the changing orientation of the stator fluxrequired to attract the rotor alternately to the horizontal and verticalpositions. The need to supply the armature winding with alternatingcurrent in such a motor can result in an excitation circuit of highcomplexity and cost.

J. R. Surano and C-M Ong, “Variable Reluctance Motor Structures forLow-Speed Operation”, IEEE Transactions on Industry Applications, Vol.32, No. 2, March/April 1996, pp 808-815 and UK Patent No. 2262843 alsodisclose fully pitched variable reluctance motors. The motor disclosedin UK Patent No. 2262843 is a three-phase variable reluctance motorhaving three windings which must be energised with current insynchronism with rotation of the rotor so that such a motor requires anexcitation circuit of high complexity.

WO 98/05112 discloses a fully pitched flux-switching motor having afour-pole stator 2 which, as shown diagrammatically in FIG. 3a, isprovided with a field winding 10 and an armature winding 11 each ofwhich is split into two coils 22 and 23 or 24 and 25 closely coupled(with a coupling which is substantially independent of rotor position)and wound so that diametrically opposite portions of both coils aredisposed within diametrically opposite stator slots. FIG. 3b shows ageneralised circuit diagram for energising the armature coils 24 and 25.The coils 24 and 25 are connected within the circuit so that directcurrent supply to the terminals 26 and 27 flows through both coils 24and 25 in the same direction so as to generate magnetomotive forces inopposite direction as a result of the opposite winding of the coils.Switches 28 and 29, which may comprise field effect transistors orthyristors for example, are connected in series with the coils 24 and 25and are switched alternately to effect alternate energisation of thecoils 24 and 25 so as to provide the required magnetomotive forcesacting in opposite directions. It is an advantage of such an arrangementthat the armature winding is made up of two closely coupled coils whichenables each coil to be energised with current in only one direction sothat relatively simple excitation circuitry can be used. A similararrangement may be provided in an electrical alternator.

GB 18027 dated Sep. 9, 1901 discloses a variable reluctance machinehaving sets of windings on the stator which are alternately energised soas to provide the required interaction with the rotor. Furthermore GB554827 discloses an inductor alternator in which the relativearrangement of the stator and rotor teeth produces successive zones ofrelatively high and low reluctance, and in which field and alternativecurrent windings are provided on the stator to effect the requiredenergisation. However, neither of these prior arrangements possesses theadvantageous feature of the closely coupled coils arrangement of WO98/05112 so that complex associated circuitry is again required.

The simplifications in the circuitry introduced by WO 98/05112 enablesimple and low cost electronic machine control, but reduce theflexibility of the machine to be controlled under rapid acceleration ordeceleration, as well as reducing the control of speed under load. It isan object of this invention to provide an electrical machine which hassimple control circuitry but can also achieve high performance.

According to the present invention, there is provided an electricalmachine comprising a rotor without windings, a stator having an armaturewinding and field magnet means for generating a magnetomotive force in adirection extending transversely of the magnetomotive force generated bythe armature winding, circuit means for controlling the current in thearmature winding such that periods in which a magnetomotive force in onedirection is associated with a first current pulse alternate withperiods in which a magnetomotive force in the opposite direction isassociated with a second current pulse, and position sensing means formonitoring the rotational position of the rotor and for supplying outputsignals dependent on the speed of rotation of the rotor, characterisedby control means for supplying control signals to the circuit means tocontrol the current in the armature winding, each control signal beingproduced in response to detection of a respective one of said outputsignals from the position sensing means and being maintained for alength of time determined by the duration of said output signal. Thearmature winding may be shunt or series connected, and the field magnetmeans may be constituted by a field winding or a permanent magnet.

The preferred embodiment of the invention allows control ofacceleration, no-load speed and loaded torque-speed characteristics tobe achieved with simple on/off control of armature and field switchingdevices, so that the appropriate control circuitry can be produced atrelatively low cost. Simplification of the control circuitry is furtherensured by the fact that such control can be effected without currentsensing. Furthermore the high frequency pulse width modulation of thecurrent pulses is required only during acceleration, thus reducingdissipation losses in the drive. The current pulse width during highspeed and loading can easily be changed to control the no-load speed andthe shape of the torque speed curve to produce a desired characteristicto match the requirement of the load.

In order that the invention may be more fully understood, reference willnow be made, by way of example, to the accompanying drawings, in which:

FIG. 1a and 1 b are explanatory diagrams showing a conventionaltwo-phase variable reluctance motor, with the two excitation modes beingshown in FIG. 1a and 1 b;

FIGS. 2a and 2 b are explanatory diagrams showing a flux-switchingmotor, with the two excitation modes being shown in FIGS. 2a and 2 b;

FIGS. 3a and 3 b are explanatory diagrams showing the stator windingsfor a flux-switching motor as disclosed in WO 98/05112;

FIG. 4 is a diagram of a flux-switching motor having an 8-pole statorand a 4-pole rotor,

FIGS. 5a, 5 b and 5 c are circuit diagrams showing circuit arrangementsfor energising the field and armature windings of embodiments of theinvention;

FIGS. 6, 7, 8 and 9 are timing diagrams showing the switch controlsignals applied during low speed operation;

FIGS. 10, 11 and 12 are timing diagrams showing the switch controlsignals applied during high speed operation;

FIG. 13 is a graph showing possible torque-speed characteristics of themotor, and

FIG. 14 is a timing diagram showing the switch control signals appliedto a further embodiment of the invention during low speed operation.

The following description of embodiments of the invention is given withreference to a flux-switching motor having a stator 2 provided witheight inwardly directed salient poles 30 and a rotor 7 having fouroutwardly directed salient poles 31 without windings, as shown in FIG.4. The stator 2 is provided with a field winding 10 and an armaturewinding 11 connected in a shunt or parallel configuration (as shown inFIG. 5a) or in a series configuration (as shown in FIG. 5c). Thearmature winding 11 may comprise two armature winding parts A1 and A2connected in series or in parallel, and the field winding 10 maycomprise two field winding parts F1 and F2 connected in series or inparallel, the winding parts being wound on the stator 2 as shown withinthe stator in FIG. 4. Each armature winding part is split into two coils24 and 25 which are closely magnetically coupled and wound so thatdiametrically opposite portions of the coils are disposed within statorslots separated by a field winding slot. The armature coils 24 and 25are wound in opposite directions and may be bifilar wound whereappropriate. However the winding configuration is preferablysubstantially as described with reference to FIG. 6 of WO 98/05112 suchthat each of the armature and field windings comprises four coils A1,A2, A3, A4 and F1, F2, F3, F4 connected together in series or inparallel (or any combination of series and parallel) and wound aroundthe stator poles such that the active portions of adjacent coils areaccommodated within the same stator slot. The winding configuration inthis case is shown in FIG. 4 by the symbols indicated outside the statorin the figure. In FIG. 4 the symbols + and − show the directions ofcurrent flow in the windings in one mode of excitation, and it will beunderstood that, in the alternate mode of excitation, the direction ofcurrent flow in the armature windings is reversed whereas the directionof current flow in the field windings is unchanged.

In the energisation circuitry 40 of the embodiment of FIG. 5a, the fieldwinding 10 is connected in parallel with the armature coils 24 and 25and a capacitor 57 which allows the currents through the field winding10 and the armature coils 24 and 25 to be different. The circuit issupplied from an alternating current source by way of a rectifier bridge59. A power MOSFET 54 and a freewheeling diode 56 are provided tocontrol the field current supplied to the field winding 10.

In the energisation circuitry 40′ of the series embodiment of FIG. 5c,the field winding 10 is connected in series with the armature coils 24,25, and a capacitor 57 is connected to the interconnection point 57 Abetween the field winding 10 and the armature coils 24, 25 so as toallow the field current to continue to flow as the energy from thearmature winding is returned back to the capacitor 57 through one of thediodes 52 or 53. A further capacitor 58 is connected across the outputof the rectifier bridge 59, and an optional inductor 60 is connected inseries with the output of the rectifier bridge 59, so as to filter thesupply to the circuit As shown in broken lines, it is also possible toprovide a diode 61 in series with the field winding 10 to prevent thecurrent in the field winding 10 reversing when the capacitor 57 ischarged to a voltage above the supply voltage on the capacitor 58. In analternative, non-illustrated arrangement, such as is shown in FIGS. 11and 12 of WO 98/05112 for example, the field winding 10 may be connectedin series with the armature coils 24 and 25. In a still further,non-illustrated arrangement, as is shown in FIG. 14 of WO 98/05112, forexample, the field winding 10 may be supplied with current from aseparate current source.

In each of these embodiments a switching control circuit is provided tosupply current alternately to the armature coils 24 and 25 so as toprovide the required magnetomotive forces acting in opposite directionsto rotate the rotor. In this case the switching control circuitincorporates two power MOSFETs 50 and 51 which are switched on and offalternately by appropriate switching pulses. Each MOSFET 50 or 51includes an integral freewheeling diode 52 or 53 so that, as each MOSFETis turned off, the stored magnetic energy in the corresponding coil iscoupled to the other coil and flows back through the freewheeling diodeof the other MOSFET. Furthermore the ends of the armature coils 24 and25 may be connected by diodes 63 and 64 to a snubber capacitor 65 whichcharges to a voltage above the supply rail voltage. The snubbercapacitor 65 is discharged by the parallel resistor 66 so as to dump theenergy stored in the snubber capacitor 65 from the imperfect switchingprocess. The snubber capacitor 65 is provided to capture energy nottransferred to the other armature coil when one of the armature coils isswitched off by its respective switching device.

The additional snubber circuit formed by the components 63, 64, 65 and66 is particularly important when insulated gate bipolar transistors(IGBTs) are used as the switching devices. IGBTs are easily damaged bydevice overvoltage, and the snubber circuit is used to contain thevoltages occurring in the circuit to a level less than the voltagerating of the IGBTs. When MOSFET's are used as in FIG. 5a, the snubbercircuit can be dispensed with if the MOSFET's are chosen to provide aninherent voltage clamp as they enter a breakdown (avalanche) mode abovetheir rated voltage. This breakdown mode absorbs the uncoupled magneticenergy associated with the imperfect coupling of the armature windingswith one another. Provided that adequate heat dissipation is availablethe MOSFET's will not suffer any damage through this process, and thecomplexity and cost of the snubber circuit is not therefore required.

On initial start-up of the motor, it is necessary to control the fieldand armature currents so as to provide the desired acceleration. Aspreviously indicated the basis of all control operations for rotation ofthe rotor is that unidirectional current is supplied to the fieldwinding substantially continuously, and alternate current pulses aresupplied to the two armature coils such that the current pulses aresynchronised to the position of the rotor. In the motor shown in FIG. 4,with eight stator poles and four rotor poles, a cycle of armatureexcitation involving positive armature mmf followed by negative mmfwould be repeated with every 90° of rotor rotation. As a result it isusual to use a rotor position sensor to control the switching transitionpoints within each armature cycle. In its simplest form the rotorposition sensor could be an optical sensor which changes polarity withevery 45° of rotation of the rotor, triggered by the interruption orreflection of an infra-red beam by the rotor or a disc mounted on therotor. Another common means of position detection would be the use of aHall effect sensor to detect north and south poles on a magnet ringattached to the rotor.

During low-speed operation, the application of the full supply voltageby turning on one of the armature switches for the whole 45° of rotationcould cause excessive armature current. The current can be controlled bypulse width modulation of the appropriate armature switch. In a shuntmotor, it may also be advantageous to pulse width modulate the switchcontrolling the field winding current so that the level of the fieldcurrent is also controlled at the same time as the armature current. Thesignal from the rotor position sensor would normally be processed by asimple microcontroller 32, as shown in FIG. 5b, which controls the gatesof the switches 50, 51 (and 54 if present) by way of gate drive circuits33. The microcontroller 32 decodes the alternating signal from theposition sensor to decide which of the switches 50 and 51 should beconducting at any point in time or perhaps that neither should beconducting. (In normal operation of the motor it is not necessary tohave both switches 50 or 51 conducting at the same time.) Themicrocontroller 32 also determines the operation of the switch 54 (ifpresent controlling the field current.

Thus, as shown by the timing diagram of FIG. 6, the change in state ofthe position sensor from low to high initiates a train of pulses on theArmature 1 output of the microcontroller 32. This train of pulses isconverted by the corresponding gate drive circuit 33 into a signalsuitable for the control of the appropriate switch 50 or 51 so as toturn the switch repetitively on and off(a switching cycle) at afrequency much higher than the sensor frequency, thus establishing andcontrolling a positive or negative armature mmf during a 45° rotation ofthe rotor. The duty cycle is the percentage of time a switch is turnedon (conducting) within each switching cycle. When the motor first startsthe duty cycle may be approximately 50%, though a value between 0 and100% can be chosen depending on the initial torque required and the rateof acceleration desired. FIG. 6 shows, at a), the 40 Hz rotor positionsignal corresponding to a speed of 600 r/min for this motor, and, at b),c) and d), the switch control signals at the Armature 1, Armature 2 andField outputs of the microcontroller 32. The pulse width modulation isapplied for virtually the whole duration of the output pulse of therotor position signal which determines the initial switch-on times ofthe switches 50 and 51. The switch control signal for the field currentis also pulse width modulated at the same time as the pulse widthmodulated switch control signals for controlling the currents in thearmature coils, and is energised continuously when the currents appliedto the armature coils are switched off.

FIG. 7 shows the form of the switch control signals supplied to thearmature coils 24 and 25 and to the field winding 10 simultaneouslyduring a part of the cycle, when the motor is operating at a speed of600 rpm. In this case a switching duty cycle of 52.5% is used to controlboth the armature and field currents. Such a duty cycle is appropriatefor control of the armature and field currents from start-up of themotor. However the duty cycle may be chosen to have any value between 0and 100% depending on the torque required and the rate of acceleration.

However, as the motor accelerates, the duty cycle of the signals appliedto the switches 50, 51 and 54 is steadily increased, and in addition theperiod of energisation of each armature coil 24 or 25 may be reduced tobe less than the total pulse width of the output of the rotor positionsignal in order to avoid excessive armature currents towards the end ofeach current energisation period of the armature coils.

When the motor has accelerated sufficiently such that the switching dutycycle has increased to close to 100%, each armature coil may be excitedfor approximately 50% of the total pulse width of the output of therotor position signal, as shown by the timing diagram of FIG. 8corresponding to a motor speed of 6,000 rpm. Motor torque andacceleration time can thus be controlled by variation of the switch dutycycle and by variation of percentage of the total pulse width of therotor position signal during which the armature is excited. In the caseof the embodiment of FIG. 4 having eight stator poles and four rotorpoles, the rotor position signal has an output frequency of 400 Hz at6000 rpm. It will be noted from this diagram that pulse width modulatedswitch control signals are supplied to the first armature coil 24 andthe field winding 10 for about 50% of the duration of the rotor positionsignal output pulse, and that the control signal is suppliedsubstantially continuously to the field winding 10 throughout theremaining 50% or so of the pulse duration. This ensures that the fieldwinding excitation is maintained at a high level which provides maximumtorque from the applied armature current. In a series motor the fieldwinding current will always be a function of the current and powersupplied to the armature and cannot be controlled independently. FIG. 9shows the switch control signals on an increased scale and indicatesthat the switching duty cycle applied for switching of the armature andfield windings is approximately 88%, that is not quite 100%.

In some applications where the load is such that rapid acceleration ofthe motor can be guaranteed (such as a pump, fan or blower in which theload torque is low at low speeds), it is possible that pulse widthmodulation of the armature switches within each polarity of armatureexcitation will not be necessary. In such a case the currents flowingmay simply be limited by the available supply conditions. The motor willthen accelerate rapidly and the current will decrease to a lower levelas the speed increases. The rate of acceleration can still be controlledby the duration and position of the armature excitation current withineach state of the position sensor.

After the motor has accelerated to the point where the switching dutycycle has reached 100%, further acceleration will result in control ofthe armature and field currents according to a high speed mode in whichpulse width modulation of the switch control signals is no longerapplied. Instead the on-time of each switch control system is reduced asthe speed is increased whilst retaining the same turn-on pointdetermined by the change in state of the rotor position signal outputpulse. This is shown by the timing diagrams of FIGS. 10 and 11. In FIG.10 the armature and field switch (if present) control signals are shownfor a motor speed of 7,500 rpm, which, in this example, is just abovethe speed at which change-over to the high speed mode is initiated. Inthis case the rotor position signal output frequency is 500 Hz, and theswitch control signals are of narrow width relative to the total widthof the rotor position signal output pulses in order to controlacceleration by limiting of the armature currents. In a shunt motorwhere control of the field is possible independently of the armature,the field switch could be turned on throughout this time, although, iflight-load efficiency is of importance, it is also possible for thefield switch control signal to be modulated to reduce field losses.

The timing diagram of FIG. 11 shows the armature and field switchcontrol signals when this particular motor is operating at a speed of9,750 rpm in which case the rotor position signal output frequency is650 Hz It will be appreciated that, in this case, the armature switchcontrol signals are narrower so as to limit the no-load speed of themotor. The circuit can be set up to provide any desired armature currentpulse width for a given motor speed to provide the required control ofno-load speed corresponding to the timing diagram of FIG. 11.Furthermore, as the switching points of the position sensor relative tothe magnetic cycle of the armature are not necessarily the same witheach implementation, the position of this narrow pulse can be placedanywhere within the position sensor cycle to obtain the best energyconversion. If a load is applied it will tend to slow down the motor andthe corresponding drop in speed may be detected as an increased lengthof time between position sensor transitions in the rotor positionsignal. As a result the on-time of the switches 50 and 51 is increasedas a function of the increasing time between rotor position signaltransitions. This function is determined either by a numericalcalculation or by reference to a look-up table relating the on-time tothe rotor position signal transitions. Variation in this function can beused to cause the torque speed curve of the motor to take on a number ofdifferent shapes. However, in all cases, the switching control is basedonly on the rotor position and the calculated speed and there is norequirement for sensing of the current to control such switching.

The motor reaches a full load point for a particular speed when thewidth of the current pulses applied to the armature coils 24 and 25 issuch that the armature circuit is excited for 100% of the availabletime. In practice, because of the inductance of the armature coils andthe finite time required to reduce each armature current pulse to zero,the maximum time of forward conduction of each switch 50 or 51 istypically between 70% and 90% of the available time. Furthermore somemotors may not operate well if the armature winding becomes continuouslyexcited. In such a case, if the current is not actively monitored, onearmature current polarity may become larger than the other armaturecurrent polarity and the motor will stall.

For this reason the on-time of the switches 50 and 51 is controlled inaccordance with the control method of the invention so as to ensure thatthe on-time is no more than a maximum predetermined percentage of theavailable time, this typically being no more than about 90% of the rotorposition signal output pulse width. This prevents instability of thearmature excitation without requiring measurement of the armaturecurrent, although some motors will not develop the imbalance describedabove and the armature switches can each operate for 100% of theposition sensor pulse width (i.e. for 50% of the armature's electricalcycle), although the current will be flowing in the diode of the deviceduring the initial part of each device conduction period. Furthermorethis method allows the motor to operate close to its natural torquespeed characteristic over a wide speed range. FIG. 12 is a timingdiagram showing the armature and field switch control signals at a motorspeed of 9,150 rpm when this method of controlling the on-time of theswitches is applied on application of a load. The corresponding rotorposition signal output frequency is 610 Hz.

The control algorithms can be designed to give a range of differentcharacteristics including delivery of a constant power characteristicover a wide speed range. Simple adjustment of the function relating theon-time of each switch 50 or 51 to the time between rotor positionsignal transitions allows any characteristic to be achieved within thenatural torque speed characteristic of the motor. FIG. 14 shows possibletorque speed characteristics which may be imparted to the motor in thismanner, the curve 70 shown in broken lines in this figure indicating thenatural torque speed characteristic of the motor with 100% armaturecurrent excitation and a given set of field winding conditions. Furtherexamples 71, 72 and 73 of possible torque-speed curves are shown forwhich torque can be controlled as a function of speed within the maximumcurve. In the region 74 such control can limit the no-load speed to anychosen value. Furthermore one of the curves 71, 72 and 73 may be chosento follow a line of constant output power over a wide speed range.

In a variant of the embodiment of the invention described above, therotor position signal is dispensed with and instead an arrangement isused for electronically calculating the position of the rotor from thesensed armature (and/or field) voltage (and/or current). This can beachieved by providing an extra winding in the armature (or field) slotsof the stator coaxial with at least one of the armature coils and bydetecting the back-emf induced in the winding. In fact the voltageinduced in this winding is a combination of the armature back-emf andthe armature supply voltage provided by the armature switches.Accordingly a suitable decoding arrangement is provided to reconstructthe back-emf waveform so as to produce a timing signal which can be usedfor timing of the armature excitation. Alternatively the armaturewinding itself can be used for such back-emf sensing since one of thearmature coils is always unenergised at any one time. Such a positionsensing arrangement enables the position of the rotor to be determinedand the excitation of the armature winding and its dependence on rotorposition to be controlled in a particularly simple manner.

It will be appreciated that, although power MOSFETs are used in theenergisation circuitry of FIG. 5a and FIG. 5c, it would also be possiblefor other types of switches to be used in the circuitry, such asthyristors and IGBT's (insulated-gate bipolar transistors).

In a further embodiment of the invention for controlling a shunt motor,the microcontroller is programed to effect continuous energisation ofthe field switch during all or part of the acceleration while themodulation of the armature switches is retained. This may provide for amore rapid acceleration of the motor and is a further simplification incontrol complexity. In the high speed mode, it is beneficial to reducethe level of current supplied to the field windings when the motor speedis close to the no-load speed. This can be implemented by switching thefield switch 54 off during the time when neither armature switch 50 or51 is turned on.

The position of the armature current pulses, and hence the rotorposition sensor, relative to the actual rotor laminations is critical toobtaining the best performance from the motor. For optimum performance,the armature mmf of positive polarity should be present when the inducedarmature voltage (due to rate of change of field flux coupling thearmature winding) becomes positive i.e. the internally induced armaturevoltage (the back emf) is in opposition to the applied armature current.As the armature winding is inductive the current takes time to changecausing a delay in the build up of current relative to theinitialisation of the control signal to the appropriate armature switch.At low speeds this time is not a significant angle of rotation but, athigh speeds, this delay may lead to a significant loss in output power.There are two ways in which this problem can be solved.

The rotor position sensor can be positioned so that the transitionsoccur close to the zero crossing of the back emf. When running at highspeed the microcontroller can use the measured speed to anticipate thesensor transitions and initiate an armature pulse in advance of thesensor transition. Such electronic advancing schemes as such are known.However, at very high speeds when the time between sensor transitionsmay be short, the accuracy obtained from such schemes can drop unless anexpensive microcontroller is employed. Such a scheme is also inaccurateat predicting the advanced turn on point if there are rapid variationsin the running speed.

Alternatively the rotor position sensor may be mechanically positioned,in advance of the zero-crossing of the armature induced voltage, suchthat at high speed the transitions of the sensor are correctlypositioned to ensure that the current has time to build up in eacharmature winding without requiring any complex control and thus allowinga simple and low cost microcontroller to be used. However such a schemehas the disadvantage that, at low speeds, the sensor transition mayinitiate a reversal of the armature mmf before it is really required.With such mechanical advancing of the position sensor, it is thereforenecessary at low speeds to delay reacting to the sensor transition untilthe rotor has turned through a further angle equivalent to the angle ofmechanical advance. In one implementation of this arrangement, amechanical advance of the position sensor of 11° was found to bebeneficial at high speed (relative to 45° sections of high and 45°sections of low). As this is approximately one quarter of the timebetween transitions it is relatively easy within a low cost digitalmicrocontroller to insert a delay of one quarter of the total measuredtime. The control signals obtained in such an arrangement are shown inFIG. 14 where the position sensor signal is shown at a), the switchcontrol signals for one armature switch are shown at b), the armaturemmf is shown at c) and the field current is shown at d).

Under light load conditions at all speeds it is not necessary to developthe full motor power and, as a result, the armature pulses may besignificantly shorter than the available time between the rotor positionsignal transitions. In such circumstances it is preferable to delay theapplication of the armature pulses even further to a time when thearmature induced voltage is at a maximum. Such a control method deliversthe maximum efficiency from the motor under light loads.

Of the above two solutions the method of mechanically advancing theposition sensor enables a simpler microcontroller to be used and ispreferred for this reason.

Finally a description will be given of the high speed and load modes ina preferred embodiment of the invention which have been developed for aparticular motor to give constant power operation when the motor isloaded from 15,000 r/min down to 7,500 r/min. It should be noted thatthese algorithms are all based on simple multiplications and divideswhich can be implemented by addition and bit-shifting in a simple lowcost microcontroller. A similar approach could be achieved with alook-up table stored in memory but this would require the storage ofmore data.

The loading of the motor causes the speed to drop from the no-load speedwhich is just above 18,000 r/min. As the speed drops a number ofsuccessive algorithms are implemented to control the pulse width appliedto each armature coil within the available time of each sensor region.For illustrative purposes, the turn-on point in all of these algorithmsis at the change of state of the sensor, that is 11° ahead of the changein polarity of the back emf, although motor efficiency may be improvedby delaying the turn-on point under some operating conditions.

The load routines are as follows:

No load speed to 15,000 rev/min. The pulse width is calculated using

((3*sensor)−267)×4 μs.

 where “sensor” denotes a count value from the microcontroller 32representing the time between transitions of the sensor.

 This algorithm increases the pulse width from the minimum value at theno-load speed and reaches a maximum pulse width of 416 μs at 15,000r/min. This corresponds to an armature excitation during 83.5% of theavailable time.

15,000 r/min to 14,300 r/min. The pulse width is rapidly shortened togive a sharp knee in the torque speed characteristic reducing from themaximum value at 15,000 r/min to 73% at 14,300 r/min. The algorithmduring this time is

(224−sensor)×4 μs.

 The negative sign in the term incorporating “sensor” ensures that thepulse width decreases in absolute time despite an increase in the valueof “sensor”.

14,300 r/min to 12,300 r/min. The algorithm during this time is$\left( {\frac{3}{16} \times {sensor}} \right) + {71 \times 4\quad \mu \quad {s.}}$

 This algorithm reduces the percentage on-time of the armature to avalue of 66% at 12,300 r/min.

12,300 r/min to 7,500 r/min. The algorithm is now changed to

(sensor/4+62) *4 μs.

 This allows a slower drop in percentage on-time as the speed decreasesfurther to maintain the operation close to constant power (17 in FIG.13).

What is claimed is:
 1. An electrical machine comprising a rotor withoutwindings, a stator having an armature winding and field magnet means forgenerating a magnetomotive force in a direction extending transverselyof the magnetomotive force generated by the armature winding, circuitmeans for controlling the current in the armature winding such thatperiods in which a magnetomotive force in one direction is associatedwith a first current pulse alternate with periods in which amagnetomotive force in the opposite direction is associated with asecond current pulse, and position sensing means for monitoring therotational position of the rotor and for supplying output signalsdependent on the speed of rotation of the rotor, characterised bycontrol means for supplying switching control signals to the circuitmeans to control the current in the armature winding, such that eachswitching control signal is produced in response to detection of arespective one of said output signals from the position sensing meansand so that the switching on time is maintained for a length of timedetermined by the duration of said output signal.
 2. A machine accordingto claim 1, wherein the armature winding comprises armature coilsconnected to the circuit means such that the currents in the coils varyin synchronism with rotation of the rotor under control of the controlmeans in such a manner that periods in which a magnetomotive force inone direction is associated with current flow in one of the coilsalternate with periods in which a magnetomotive force in the oppositedirection is associated with current flow in the other coil.
 3. Amachine according to claim 2, wherein the coils are closely coupledmagnetically.
 4. A machine according to claim 2, wherein the circuitmeans comprises respective switch means for alternately conducting firstcurrent pulses in one of the armature coils and second current pulses inthe other armature coil under the control of the control means.
 5. Amachine according to claim 1, wherein the field magnet means comprises afield winding wound on the stator and supplied with substantiallyunidirectional current by the circuit means.
 6. A machine according toclaim 5, wherein the field winding is connected in a parallelconfiguration with the armature winding.
 7. A machine according to claim5, wherein the field winding is connected in a series configuration withthe armature winding.
 8. A machine according to claim 1, wherein, in alow speed mode, the control means is arranged to produce pulse widthmodulated switching control signals having a duty cycle which increaseswith increasing speed of the rotor to control the current in thearmature winding when the rotor is rotating at a relatively low speed.9. A machine according to claim 8, wherein, in the low speed mode, thecontrol means is also arranged to produce pulse width modulatedswitching control signals to control the current in the field magnetmeans.
 10. A machine according to claim 1, wherein, in a high speedmode, the control means is arranged to produce switching control signalswhich are not pulse width modulated to control the current in thearmature winding when the rotor is rotating at a relatively high speed.11. A machine according to claim 10, wherein, in the high speed mode,the control means is arranged to produce switching control signals ofdecreasing width as the speed of rotation of the rotor increases.
 12. Amachine according to claim 10, wherein, in the high speed mode, thecontrol means is arranged to produce switching control signals having aswitching on-time which is controlled to be less than a maximumpredetermined value when a load is applied to the machine.
 13. A machineaccording to claim 1, wherein the control means is arranged to produceswitching control signals having a switching on-time which is controlledso as to follow a predetermined torque-speed characteristic.
 14. Amachine according to claim 1, wherein the position sensing meansincorporates a sensor for providing an electrical output in response todetection of markings on the rotor.
 15. A machine according to claim 1,wherein the position sensing means incorporates a sensor for detectingvariation of a parameter of the armature winding or the field magnetmeans or a separate sensor winding provided on the stator.
 16. A machineaccording to claim 15, wherein the armature winding incorporates thesensor winding.
 17. A machine according to claim 1, wherein the positionsensing means incorporates decoding means providing a timing signalrelated to the position of the rotor.